Objective Data on the aetiology of persistent cough at the transitional stage from subacute to chronic cough (>4 weeks duration) are scarce. We aimed to (1) identify the prevalence of chronic cough following acute respiratory illness (ARI) and (2) determine the diagnostic outcomes of children with chronic cough.

Design Prospective cohort study.

Setting A paediatric emergency department (ED) in Brisbane, Australia.

Patients Children aged <15 years presenting with an ARI with cough.

Interventions Children were followed weekly for 28 days;those with a persistent cough at day 28 were reviewed by a paediatric pulmonologist.

Main outcome measures Cough persistence at day 28 and pulmonologist diagnosis.

Results 2586 children were screened and 776 (30%) were ineligible; 839 children (median age=2.3 years, range=0.5 months to 14.7 years, 60% male) were enrolled over 2 years. Most children (n=627, 74.8%) had cough duration of <7 days at enrolment. At day 28, 171/839 (20.4%, 95% CI 17.7 to 23.1) children had persistent cough irrespective of cough duration at enrolment. The cough was wet in 59/171 (34.5%), dry in 45/171 (26.4%) and variable in 28/171 (16.1%). Of these 117 children , 117 (68.4%) were reviewed by a paediatric pulmonologist. A new and serious chronic lung disease was diagnosed in 36/117 (30.8%) children; 55/117 (47.0%) were diagnosed with protracted bacterial bronchitis.

Conclusions When chronic cough develops post-ARI, clinical review is warranted, particularly if parents report a history of prolonged or recurrent cough. Parents of children presenting acutely to ED with cough should be counselled about the development of chronic cough, as an underlying respiratory condition is not uncommon.